'\"
'\" Copyright (c) 1992-1994 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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.TH Tcl_BackgroundError 3 7.5 Tcl "Tcl Library Procedures"
.so man.macros
.BS
.SH NAME
Tcl_BackgroundException, Tcl_BackgroundError \- report Tcl exception that occurred in background processing
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
\fBTcl_BackgroundException\fR(\fIinterp, code\fR)
.sp
\fBTcl_BackgroundError\fR(\fIinterp\fR)
.SH ARGUMENTS
.AS Tcl_Interp *interp
.AP Tcl_Interp *interp in
Interpreter in which the exception occurred.
.AP int code in
The exceptional return code to be reported.
.BE

.SH DESCRIPTION
.PP
This procedure is typically invoked when a Tcl exception (any
return code other than TCL_OK) occurs during
.QW "background processing"
such as executing an event handler.
When such an exception occurs, the condition is reported to Tcl
or to a widget or some other C code, and there is not usually any
obvious way for that code to report the exception to the user.
In these cases the code calls \fBTcl_BackgroundException\fR with an
\fIinterp\fR argument identifying the interpreter in which the
exception occurred, and a \fIcode\fR argument holding the return
code value of the exception.  The state of the interpreter, including
any error message in the interpreter result, and the values of
any entries in the return options dictionary, is captured and
saved.  \fBTcl_BackgroundException\fR then arranges for the event
loop to invoke at some later time the command registered
in that interpreter to handle background errors by the
\fBinterp bgerror\fR command, passing the captured values as
arguments.
The registered handler command is meant to report the exception
in an application-specific fashion.  The handler command
receives two arguments, the result of the interp, and the
return options of the interp at the time the error occurred.
If the application registers no handler command, the default
handler command will attempt to call \fBbgerror\fR to report
the error.  If an error condition arises while invoking the
handler command, then \fBTcl_BackgroundException\fR reports the
error itself by printing a message on the standard error file.
.PP
\fBTcl_BackgroundException\fR does not invoke the handler command immediately
because this could potentially interfere with scripts that are in process
at the time the error occurred.
Instead, it invokes the handler command later as an idle callback.
.PP
It is possible for many background exceptions to accumulate before
the handler command is invoked.  When this happens, each of the exceptions
is processed in order.  However, if the handler command returns a
break exception, then all remaining error reports for the
interpreter are skipped.
.PP
The \fBTcl_BackgroundError\fR routine is an older and simpler interface
useful when the exception code reported is \fBTCL_ERROR\fR.  It is
equivalent to:
.PP
.CS
Tcl_BackgroundException(interp, TCL_ERROR);
.CE

.SH KEYWORDS
background, bgerror, error, interp
